Each document, reference, patent application or patent cited in this text is expressly incorporated herein in their entirety by reference, which means that it should be read and considered by the reader as part of this text. That the document, reference, patent application, or patent cited in this text is not repeated in this text is merely for reasons of conciseness.
The following discussion of the background to the invention is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge of the person skilled in the art in any jurisdiction as at the priority date of the invention.
RFID involves readers (also referred to as interrogators) and tags (also referred to as cards or labels). RFID tags are devices operable to send data such as, for example, an identification (a “tag ID”) to an RFID, reader for identification purposes.
In operation, a reader will attempt to communicate with a tag within the reader's transmission area or field. The reader is operable to transmit a predetermined signal (in the transmission area or field) and then monitors the signal. A tag responding to the signal is operable to modulate it in a predetermined manner which is identified by the reader.
FIG. 1 of the drawings depicts a conventional arrangement of an RFID system 10 comprising a typical low frequency (125 KHz) RFID reader 12 having a tuned loop reader antenna 14 operable to receive a response signal from a typical tag 16. FIGS. 2A and 2B of the drawings depict simplified block diagrams for the reader 12 and the tag 16, respectively.
The reader 12 comprises a reader microprocessor 18 operable to provide a stable 125 KHz reference frequency from an onboard pulse-width modulation (“PWM”) output. This is amplified by an un-modulated RF reader amplifier 20 and used to power the reader antenna 14 at a frequency of 125 KHz. Current in the loop of the reader antenna 14 generates an inductive alternating current (“AC”) field around the loop. Also connected to the loop of the reader antenna 14 is an envelope detector 22, the simplest of which may have the form of a diode detector. Output from the envelope detector 22 is presented to a detector amplifier 24. This is depicted in FIG. 2A as being operably connected or going to an analog-to-digital converter (“ADC”) 26, but its output could be taken to a comparator in simple readers. The components of the reader 12 are operably connected such that any signal modulation that appears on the tuned loop of the reader antenna 14 will be detected and amplified.
The tag 16 comprises a tuned loop tag antenna 28 operably coupled to a tuned circuit. A tag rectifier 30 is provided and is operable to tap off some of the power in the tuned circuit to power or run a tag microchip 32. The tag 16 further comprises a clock extractor 34 operable to divide the RF frequency by a factor, which may be, for example, 32, to provide an output data rate, a 64 bit shift register 36 containing the tag data, and a tag modulator 38 operable to modulate the tuned loop of the tag antenna 28. When the tag 16 is placed in an RF field (such as the transmission area or field) generated by the reader 12, a voltage on the tuned circuit of the tag 16 increases or builds up until the tag rectifier 30 is operable to supply enough power for the tag microchip 32 to work or function, that is energize the tag 16. A typical tag will have 64 bits of stored data in the shift register 36, although there are many different tags available with memories storing varied amounts of data from just a few bits to many thousands of bits. For a better understanding, the tag may use the clock extractor 34 to divide down the 125 KHz frequency by 32 and use this as a reference frequency. Typically this reference frequency can be used as a clock to rotate the shift register 36 containing the tag data, such as the tag ID. The shift register 36 is arranged to rotate the 64 bits of data around and around in a continuous loop. A serial output of the shift register 36 is used to modulate RF voltage on the receiver coil of the tag 16. The data is usually converted into Manchester or Bi-phase encoding to ensure that the signal has no direct current (“DC”) component. A typical waveform in this regard is depicted in FIG. 3 of the drawings.
The tuned loop of the tag antenna 28 is coupled into the tuned loop of the reader antenna 14 such that the modulation of the tag 16 also appears on the tuned loop of the reader antenna. This modulated signal can be several tens of μV to several tens of mV depending on the distance between the tag 16 and the reader 12. By operation of the envelope detector 22 and detector amplifier 24 of the reader 12, the signal is detected and amplified and presented to the ADC 26 and then subsequently to the microprocessor 18. Many microprocessors have internal ADCs. In a normal or traditional (non-anti-collision system) tag reader, the analog to digital is used to detect when the signal is positive or negative compared to a no signal voltage, allowing for the received tag data to be decoded back, from the encoded Manchester code, for example, to raw data. Many readers also use a standard integrated circuit (“IC”) comparator in this position and present the output to a microprocessor port for decoding and processing.
Often the detected signal is amplified until it limits, rail to rail, and this can make detection easier. Typically the received waveform can be compared to a centre rest voltage with a comparator or digitally using an ADC and subtracting samples. The timings between switching are compared and the associated bit, ‘0’ or ‘1’, chosen that corresponds best to the particular encoding of the tag data.
As a general rule, an RFID system such as that described above works well. Cards or tags and readers are typically inexpensive and to date, this system is the widest in use of all card/tag systems and is used for many applications, including asset tracking, door entry, logistics, and maintenance.
Modern 125 kHz RFID readers consume relatively low power compared to their counterparts a decade ago. This has been due to steady improvements in technology including, for example, processor efficiency, operational amplifiers and RF power semiconductors.
However, improvements in power consumption have now leveled off so that significant reductions below the current level are unlikely to be achieved using prior art technology and methods, especially since the bulk of the power consumed in present day RFID readers is used to drive the search coil and this cannot be reduced without increasing the Q of the search coil of the reader and this raises other problems, none the least cost.
Additionally, present day power levels are still preventing the extension of RFID technology to new areas. Applications remain largely the same as they did a decade ago because RFID readers are constrained by the use of mains powered supplies, or in the case of portable readers, batteries with considerable power reserves.
To permit applications that are powered by inexpensive solar cells or by lithium coin cells, present-day power levels present a formidable barrier. For example, a typical reader will run at 12V and draw 40 mA using 400 mW-500 mW of power. At this rate, four off lithium CR2477 1000 mA/hr coin cells would last just one day, and this is nowhere near the duration required or desired, such as at least a year or so, that would be acceptable for installations, such as sheds, safes, medical cabinets, boats, caravans and the like. Improvements in the order of at least 100× are predicted to be required before the technology can really make significant inroads into these areas.
It is against this background that the present invention has been developed.